With the development of electronic technology, particularly information-related technology typified by computers, the demand for information recording media such as magnetic disks, optical disks, and magneto-optical disks has increased greatly. The principal structural components of the magnetic recording devices of computers and the like are magnetic recording media and magnetic heads for magnetic recording and reproduction. Magnetic recording media in the forms of flexible disks and hard disks are known. Hard disk (magnetic disk) substrates may be made of a variety of materials; there are aluminum, glass, ceramic, and carbon substrates. In practical terms, aluminum substrates and glass substrates are primarily employed depending on size and the application involved. However, as the hard disk drives of notebook computers have decreased in size and the density of magnetic recording has increased, the requirements of surface smoothness and thickness reduction in disk substrates have become ever more stringent. Therefore, the limitations of aluminum substrates, with their poor processability, strength, and rigidity, have been revealed. Accordingly, glass substrates for magnetic disks having good strength, rigidity, impact resistance, and surface smoothness have appeared in recent years (for example, see Japanese Examined Patent Publication (KOKOKU) Showa No. 47-1949 and Japanese Unexamined Patent Publication (KOKAI) Heisei Nos. 5-32431 and 10-1329, which are expressly incorporated herein by reference in their entirety).